Many different types of lighting aids and systems have been devised and used to provide emergency lighting for use when conditions render normal ambient lighting insufficient for visibility. Such conditions include power outages, smoke caused by fires, water immersion, and chemical fog.
One such system operable upon aircraft water immersion is disclosed in U.S. Pat. No. 4,597,033 to Meggs et al and assigned to the assignee herein. This system utilizes light emitting diodes (L.E.D.s) to form a strip which illuminates the outline of an egress hatch in a helicopter when it is submerged in water and is effective in conditions of considerable turbidity.
Another system, shown in U.S. Pat. No. 4,682,147 to Bowman, utilizes a plurality of L.E.D.s in an "EXIT" sign. The sign is illuminated during power failure to indicate a means of egress to confined occupants.
"EXIT" signs are a common sight in theaters, office buildings, stores, subways and other confining structures frequented by the public. Such signs are commonly illuminated by conventional incandescent light bulbs or, as illustrated in the Bowman patent, by L.E.D.s. The light sources in these signs emit their light spherically or hemispherically. Adequacy of these signs is frequently measured by their brightness, which is often equated to their visibility under emergency conditions.
However, this brightness standard is now being questioned, since, under conditions of visibility impaired by the presence of smoke or fog, this brightness can become a handicap. This is caused by diffusion of the light by the particles comprising the smoke or fog. As the distance between the observer and the lighted sign increases, the distinctness of the letters, then the sign itself, rapidly diminishes until only a general glow is distinguishable, which does not indicate the emergency exit that is the source of the light.
Also, as distance from the sign increases, the light scattered by the particulate matter, the light source becomes indistinguishable from the scattered light. Thus, the light source is no longer discernable at all. Unfortunately, a significant increase in light intensity increases the range of visibility only slightly. Thus, a very bright conventional sign becomes useless at a very short distance from it in dense smoke conditions which severely limits visibility.
Light strips are now being provided along aisles of airplanes to mark an emergency egress route. These comprise spaced light sources that suffer the same general visibility problems as the signs, being subject to rapid light diffusion in smoke. These light strips improve over signs, since they provide a light source that is closer to aircraft occupants by extending the length of the aisle.
However, in dense smoke conditions, the lighted strip rapidly visually disappears as viewing distance increases and transitions through a series of glowing pinpoints of light until only one pinpoint is visible at a time. If only a single light pinpoint is discernable to an occupant, the location of the next pinpoint of light can only be guessed and the strip of light loses its primary function of marking a path to an emergency exit.
There is a need for emergency lighting that better penetrates smoke and fog to provide a visible and discernable guide to occupant egress from a confined area during an emergency which is functional at distances much greater than present lighting systems.